For private homes and also local area networks (LAN) developments are going on to connect all kinds of devices as TV, PC, stereo system, alarm system, telefone, etc. together. Known are already home systems which communicate by using the 230 V power line.
The object of the invention is to provide a time reference system and a synchronisation method for a receiver of above indoor communication system.
All devices of a home have to work in a quasi-synchronous mode with a frequency stability for example of about 10xc3x9710xe2x88x926. The inventive transmitted signals of any device are frame-aligned to the signals of the others. The timing of the following frame is basedxe2x80x94according to the inventionxe2x80x94directly on the last preceding control slot signal. The transmitted control slot signals are related to the respective preceding control slot, the first control slot of the frame is related to the last control slot of the previous frame. The timing of the data slot signals of all devices will be based on the last control slot signal of the same frame, applying the defined frame structure. At the end of the frame a longer guard-time can be included.
A receiverxe2x80x94according to the inventionxe2x80x94for the frequency synchronisation of a device is described. To synchronize on different signals within a frame which are incorrect in their relative timing and differ in their RF frequency more or less, an ad-hoc process is necessary. From the correlation of the midamble a first correlation signal is derived. With this first correlation signal a channel-equalization-process is started which yields a channel corrected output signal. From this channel corrected output signal the signal identically to the input signal can be reconstructed by using the channel pulse response.
relevant only for the signals within a cluster, and in some respect also for the identification of signals from other clusters.
1.1 Introduction
The various signals of the users or terminals of one cluster are embedded in a common TDMA frame. If the use of a second channel is allowed, this embedding applies separately to both channels, whilst the messages and data transmission of a user may be split over the two channels. The inventive frame organization is based on time slots which are assigned to the users during initialization processes and which requires at least a quasi-time-synchronous generation and transmission of the slot signals. Furthermore, certain timing tolerances have to be kept in order to avoid collisions within the own frame and to ensure transmission of the defined data rates. In addition, the evaluation of the various signals in the receiver should be as simple as possible where especially the maximal centre frequency deviation could be an important factor, e.g. for the correlation with midambles and channel equalization process.
The principles are described and tolerance proposals are given in the chapters 1.2. and 1.3. Especially the tolerances may be subject to further considerations.
Transmissionxe2x80x94especially the correct positioning of the transmitted slot signalsxe2x80x94requires an adequate receiver in order to monitor and time-evaluate the signals of the other users and accordingly arrange the own signals. The corresponding part is described in the chapters 2. xe2x80x98Receiver synchronizationxe2x80x99 and 3. xe2x80x98Channel acquisition, monitoring and sensing processesxe2x80x99.
Since the various signals of the frame are not fully synchronousxe2x80x94neither in the RF frequency nor in the timingxe2x80x94, the receiver has to deal with changing conditions and a quasi-ad-hoc. synchronization or evaluation must be performed if signals of more than one user shall be monitored or evaluated.
1.2 RF (frequency) accuracy
Depending on the requirements, the reference oscillator(s) can be a remarkable cost factor. On the other hand, a very complex processing in the receiverxe2x80x94in order to deal with rather bad conditionsxe2x80x94might also cause realization problems. Finally, a compromise has to be made.
A requirement from the receiver point of view could be to allow a fast monitoring with practicable means, i.e. with only one correlation per slot or midamble, which implies that the phase rotation within the midamble caused by the frequency deviation is significantly less than 180xc2x0, otherwise additional correlations with pre-distorted midambles have to be performed or the result in critical cases will be significantly less than the maximum.
Taking into account the relatively high frequencies of the ISM bands with 2.4 and 5.7 GHz and furthermore the fact that the receiver has to work with corresponding deviations of the transmitted signal and of its own oscillators, this leads to a tolerance of xc2x110*10xe2x88x926. The corresponding frequency and phase deviations are shown in Table 1.1.
The deviations over a few symbols are already too big to be ignored in the receiver. Dedicated maximum search and pulse response evaluation methods and also special 10 signal corrections methods or equivalent measures are needed in order to achieve sufficient results in the correlation and demodulation processes. On the other hand, a crystal oscillator with an accuracy and long-term stability of xc2x110 ppm is already a sophisticated device for consumer applications and it seems not to be adequate to place more severe constraints on this part of the system.
1.3 Frame synchronization and timing of slots and symbols
The transmitted signal(s) of any user must be frame-aligned to the signals of the others (if present), which implies:
Positioning and numbering of the own control slot signal in accordance with the positioning and numbering of the control slot signals of the other users (preferably the one being ahead in the series of signals)
In additionxe2x80x94to concatenate all cluster signals and avoid the formation of subclusters xe2x80x94, the timing of any control slot signal shall directly be based on the preceding control slot signal, transmitted by another user or, if no other user exists, by the own transmitter in the previous frame; normally a signal within the same frame but in case of the first one this has to be the last control slot signal from the preceding frame.
The timing of the data slot signals of all users will be based on the last control slot signal having relevant amplitude, applying the defined frame structure.
It has to be noted that, under certain conditionsxe2x80x94acquisition phasexe2x80x94control signals may not be transmitted in all frames; the principles are described in the System Description. This requires a flexible operation of the device with respect to the slot signal to be used as reference.